The present invention relates to an apparatus and method of purifying the nitric oxide in the exhaust gas of a diesel engine, and more particularly to an apparatus for purifying the nitric oxide in the exhaust gas of a diesel engine that is capable of purifying the nitric oxide in the exhaust gas efficiently at low temperature.
Generally, a catalyst converter, through which exhaust gas passes, is mounted on an exhaust gas pipe, or an exhaust manifold, of an internal combustion engine. The catalyst converter converts the harmful materials in the exhaust gas, such as CO, HC, and NOx, into CO2, H2O, and N2.
Among the harmful materials in the exhaust gas is NOx. NOx is a general term for a nitric oxide such as NO, NO2, N2O. Air contains about 77% nitrogen and nitrogen is not easily oxidized under normal conditions. But nitrogen is oxidized under the high pressure and the high temperature in the combustion chamber of the internal combustion engine and becomes nitric oxide.
Nitric oxide injures the eyes and the lungs of people and causes the photochemical smog. Therefore, the nitric oxide contained in the exhaust gas of an automobile should be reduced or removed.
Additionally, a large amount of CH, CO, and NOx is discharged in the exhaust gas of a diesel engine. Therefore, the technical investigation to reduce or remove CH, CO, and NOx has proceeded.
Prior methods of removing the NOx include a quaternary catalyst reduction method, a hydrocarbon selection catalyst reduction method and a urea selection catalyst reduction method.
Because the quaternary catalysts have a low nitric oxide purifying efficiency (10% on a chasis dyno) the quaternary catalysts are not effective.
The hydrocarbon selection catalyst reduction method has a nitric oxide purifying efficiency of 35% or so on a chasis dyno, and does not have a high nitric oxide purifying efficiency even though the penalty of the fuel efficiency is 3%.
The urea selection reduction method has a nitric oxide purifying efficiency of not less than 65%. Therefore, the urea selection reduction method is considered the preferred method for purifying nitric oxide.
However, a urea selection reduction system using the urea selection catalyst method has not been utilized, because a suitable sensor for nitric oxide has not been developed until now. A urea injection system is operated depending on the output of the sensor for sensing several input data.
Furthermore, a general SCR (Selective Catalytic Reduction) and a NOx adsorber are commonly used to reduce the amount of the nitric oxide in the exhaust gas. A Pt/Al2O3 catalyst is commonly used as the SCR.
To satisfy the Europe Safety Regulation of nitric oxide, the nitrogen oxide purifying efficiency must be increased. Therefore new technology to reduce nitrogen oxide emissions is required.
According to the Europe Safety Regulation of nitric oxide, the particular amount of nitric oxide discharged at a particular temperature of exhaust gas, for traveling a particular unit distance, are described in table 1.
As shown in Table 1, about 50% of the nitric oxide is discharged under a temperature of 200xc2x0 C.
The activation temperature of the SCR (ex, Pt/Al2O3) is above 200xc2x0 C., so the SCR cannot satisfy the Europe Safety Regulation of nitric oxide.
A NOx adsorber is mainly used in a Lean-burn Engine. Therefore, it is difficult to apply the NOx adsorber to a diesel engine maintained under an environment of higher oxygen.
The NOx adsorber occludes the NOx until the temperature of the exhaust gas reaches 200xc2x0 C., and then the NOx is removed by activating the NOx adsorber. The activation of the NOx adsorber is accomplished by controlling the theoretical airxe2x80x94fuel ratio to 14.7. But in a diesel engine, it is very difficult to adjust the theoretical airxe2x80x94fuel ratio to 14.7.
To satisfy the Europe Safety Regulation of nitric oxide, at least 50% of the NOx contained in the exhaust gas of an automobile should be removed.
The object of the present invention is to provide an apparatus and method of purifying the nitric oxide contained in the exhaust gas of a diesel engine, which can increase the nitric oxide purifying efficiency to at least 50% or more.
To achieve the above object, the present invention comprises an NOx adsorber converter mounted on an exhaust gas pipe close to the engine, a urea catalyst converter mounted on the exhaust gas pipe at some distance from the NOx adsorber converter, and a urea supplying means connected between the NOx adsorber converter and the urea catalyst converter.
The urea supplying means comprises a tank full of urea, a feed pump for feeding the urea in the tank to the urea catalyst converter, an injector for discharging the urea mixed with air into the urea catalyst converter, a temperature sensor for sensing the temperature of the exhaust gas pipe, and a controller driving the feed pump based on the temperature of the sensor being higher than predetermined level.
The apparatus in accordance with the present invention occludes the NOx until the temperature of the exhaust gas reaches 200xc2x0 C. using a BaO catalyst in the NOx adsorber converter. The NOx occluded in the converter is separated from the BaO catalyst and discharged.